Foundation structure, which can be installed with a low level of noise, for an offshore plant, in particular an offshore wind energy plant, and assembly method therefor.
The present invention relates to a foundation structure for an offshore plant, in particular an offshore wind energy plant, having at least one foundation element which can be anchored to the sea floor, precluding a gravity foundation and precluding a floating foundation, and a support structure which can be attached thereto to fix the offshore plant. The invention also relates to a method for assembling this foundation structure on a sea floor.
In the context of the present invention, the term ‘offshore plants’ encompass offshore platforms and offshore wind energy plants. Offshore platforms also include so called drilling islands. Generally known foundations for offshore plants, in particular offshore wind energy plants (OWEPs), can be divided into two construction areas. The first construction area is a support structure which is highly subject to fatigue and which begins on the sea floor and has its end reaching the tower of the OWEP at a flange connection. The flange connection supports the tower and a turbine of the OWEP and dissipates the loads and effects produced thereby. The second construction area is a foundation element which receives the loads arising from the support structure, the tower and the wind turbine, dissipates them into the floor and is located in the sea floor below the support structure. The OWEP as a whole therefore consists of the foundation with the foundation element and the support structure and the tower and the turbine.
In order to provide a foundation for an OWEP in the sea floor the use of so-called piles as foundation elements is known from DE 20 2010 010 094 U1, these foundation elements having a diameter of about 1.5 m to about 6 m depending on the construction. The number of piles used is dependent on the support structure concerned. According to the current state of the art, various steel constructions are known as support structures, being provided with a foundation of piles: monopile, jacket, tripile and tripod.
A jacket support structure known from EP 2 067 913 A2 is attached to the sea floor by four piles, while tripiles and tripods require piles with a larger diameter. For a monopile, a single pile will suffice if it has a substantially larger diameter than other foundations. Furthermore, from the document DE 20 2011 101 599 U1 a hexagonal support structure is known, having a foundation of six piles.
Depending on the construction and the properties of the floor, the piles are driven up to 65 metres down into the sea floor. They have a weight of about 220 to 700 tonnes depending on the properties of the floor and the support structure selected. The lower ends of the support structure, the support structure feet, can be connected to the foundation piles in such a way that the feet of the support structure are pushed into the driven foundation piles which have a larger diameter. The foundation pile and support structure foot are then connected to each other e.g. by a special cement mixture (grout compound).
In addition to the foundation option using piles, gravity foundations are also known as foundations for offshore wind energy plants from DE 10 2010 012 094 B3. These consist of reinforced concrete and can weigh up to about 7000 tonnes.
Furthermore, from the international patent application WO 2011/030167 A1, attachment of turbines in tidal power generation plants below sea level by means of grouted piles bored vertically into the sea floor is known. The use of grouted piles in relation to offshore wind energy plants is described in the American patent application US 2011/0061321 A1. However, the grouted piles are used together with a gravity foundation as a kind of hybrid solution.
Furthermore, the British patent GB 880 467 describes the use of a driven piles to attach lattice mast-like foundation structures to the sea floor. The American patent application US 2011/0293379 A1 discloses lattice mast-like anchoring structures which are connected to the sea floor by means of grouted piles and to which mooring lines for floating offshore plants are connected in the manner of a floating foundation.
The approval process for erecting offshore wind farms has a profound influence on the selection of the techniques to be used offshore. In Germany, the Bundesamt fur Seeschifffahrt and Hydrografie (BSH) is responsible for the approval process. In addition, the Bundesamt fur Naturschutz (BfN) enters the approval process whenever questions and conflicts arise in relation to environmental protection.
With respect to the support structures and foundation elements, it is decided during the approval process, whether the operator's proposed construction formed from a foundation element and support structure can be used. Environmentally significant considerations and also technical requirements are of significance in the decision-making process. In designing OWEPs, attention is paid in particular to the most environmentally friendly solutions possible, which avoid or reduce sealing of the sea floor.
There are currently essentially six different foundation structures, monopile, gravity foundation, tripod, tripile, jacket and a floating anchorage, which compete with each other depending on the depth of the water. With the exception of the gravity foundations, all structures are usually provided with pile foundations. The heavy pile hammers used for the pile-driving process cause considerable sound emissions and shaking emissions. These emissions are output both in the air and also in the water and are to the considerable detriment of the natural world and the environment.
Endangered species include, amongst others, fish stocks, common seals and grey seals, whales and porpoises and sea floor fauna (benthos). The limit for acoustic pressure is currently 160 dB at 750 m from the source of the emission. However, this value is generally clearly exceeded during pile-driving.
Owing to the impact on the marine environment, wind farm operators and installation companies are obliged to use sound-damping measures during pile-driving work.
However, these are currently still in the trial phase. So-called quenching water, amongst other things, is used. However, it is not yet clear whether the sound damping is sufficient to remain below the limit value. Furthermore, quenching water is affected by sea currents so that its sound-damping effect is reduced. The use of quenching water is also time-consuming and expensive and is therefore uneconomical.
Gravity foundations are also critically evaluated with respect to environmental compatibility. Gravity foundations are produced from reinforced concrete and—owing to their shape and operating principle—take up a very large amount of surface area compared with other solutions and seal the sea floor in the region of the foundation, the sea floor then being no longer usable to sea floor fauna and flora. In addition, such foundations require elaborate and time-consuming preparation of the sea floor. Therefore, foundations using piles and no gravity foundations are generally provided in offshore wind energy plants.
For static and dynamic calculations of the loading effects on the sea floor, geological investigations are also required at each potential location. These are time-consuming and expensive.
A further important point in the approval process is the assurance that the OWEPs can be removed after use. The duration of use of an OWEP is given as 20 to 25 years. After shut-down, it has to be dismantled, wherein the piles or gravity foundations have to be removed.